Integrated circuits are made by photolithographic processes which use photomasks or reticles and an associated light source to project a circuit image onto a silicon wafer. A high production yield is contingent on having defectless masks and reticles. Since it is inevitable that defects will occur in the mask, these defects have to be found and repaired prior to using the mask.
Automated mask inspection systems have existed for over 15 years. The earliest such system, the Bell Telephone Laboratories AMIS system (John Bruning et al., "An Automated Mask Inspection System--AMIS", IEEE Transactions on Electron Devices, Vol. ED-22, No. 7 July 1971, pp 487 to 495), used a laser that scanned the mask. Subsequent systems used a linear sensor to inspect an image projected by the mask, such as described by Levy et al. (U.S. Pat. No. 4,247,203, "Automatic Photomask Inspection System and Apparatus") who teach die-to-die inspection, i.e., inspection of two adjacent dice by comparing them to each other. Alternately, Danielson et al. (U.S. Pat. No. 4,926,489, "Reticle Inspection System") teach die-to-database inspection, i.e. inspection of the reticle by comparison to the database from which the reticle was made.
As the complexity of integrated circuits has increased, so has the demand on the inspection process. Both the need for resolving smaller defects and for inspecting larger areas have resulted in much greater speed requirements, in terms of number of picture elements per second processed. The increased demands have given rise to improvements described in a number of subsequently issued patents, such as U.S. Pat. No. 4,247,203, entitled "Automatic Photomask Inspection System and Apparatus", Levy et al., issued Jan. 27, 1981; U.S. Pat. No. 4,579,455, entitled "Photomask Inspection Apparatus and Method with Improved Defect Detection" Levy et al , issued Apr. 1, 1986; U.S. Pat. No. 4,633,504, entitled "Automatic Photomask Inspection System Having Image Enhancement Means", Mark J. Wihl, issued Dec. 30, 1986; and U.S. Pat. No. 4,805,123, entitled "Automatic Photomask Inspection and Reticle Inspection Method and Apparatus Including Improved Defect Detector and Alignment Subsystem", Specht et al., issued Feb. 14, 1989. Also of relevance is some prior art in the wafer inspection area, such as U.S. Pat. No. 4,644,172, entitled "Electronic Control of an Automatic Wafer Inspection System", Sandland et al., issued Feb. 17, 1987.
Another force driving the development of improved inspection techniques is the emergence of phase shift mask technology. With this technology it will be possible to print finer line widths, down to 0.25 micrometers or less. This technology is described by Burn J. Lin, "Phase-Shifting and Other Challenges in Optical Mask Technology", Proceedings of the 10th Annual Symposium on Microlithography, SPIE,--the International Society of Optical Engineering, Vol. 1496, pages 54 to 79.
The above improvements teach the automatic detection of defects on conventional optical masks and reticles. In all of these systems, conventional lighting is used and the images are captured by linear array sensors. These two system choices limit the signal-to-noise ratio and hence the speed of inspection.
Additionally, a photomask is used in the semiconductor manufacturing industry for the purpose of transferring photolithographic patterns onto a substrate such as silicon, gallium arsenide, or the like during the manufacture of integrated circuits. The photomask is typically composed of a polished transparent substrate, such as a fused quartz plate, on which a thin patterned opaque layer, consisting of figures, has been deposited on one surface. Typically the patterned opaque layer is chromium with a thickness of 800 to 1200 angstroms. This layer may have a light anti-reflection coating deposited on one or both surfaces of the chromium. In order to produce functioning integrated circuits at a high yield rate, the photomasks need to be free of defects. A defect is defined here as any unintended modification to the intended photolithographic pattern caused during the manufacture of the photomask or as a result of the use of the photomask. Defects can be due to, and not limited to, a portion of the opaque layer being absent from an area of the photolithographic pattern where it is intended to be present, a portion of the opaque layer being present in an area of the photolithographic pattern where it is not intended to be, chemical stains or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, particulate contaminates such as dust, resist flakes, skin flakes, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or opaque layer. During the manufacture of photomasks, automated inspection of the photomask is performed in order to ensure a freedom from the aforementioned defects.
There are, at present, two methods for the inspection of patterned masks or reticles. One of those inspection methods is die-to-die which uses transmitted light to compare either two adjacent dies or a die to the CAD database of that die. These comparison-type inspection systems are quite expensive because they rely on pixel-by-pixel comparison of all the dies and, by necessity, rely on highly accurate methods of alignment between the two dies used at any one time for the comparison. Apart from their high costs, this method of inspection is also unable to detect particles on opaque parts of the reticle which have the tendency to subsequently migrate to parts that are transparent and then cause a defect on the wafer. This method of inspection is described in U.S. Pat. Nos. 4,247,203 and 4,579,455, both by Levy et al.
The second method of the prior art for inspection of patterned masks is restricted to locating particulate matter on the mask. It makes use of the fact that light scatters when it strikes a particle. Unfortunately, the edges of the pattern also cause scattering and for that reason these systems are unreliable for the detection of particles smaller than 1 micrometer. Such systems are described in a paper entitled "Automatic Inspection of Contaminates on Reticles" by Masataka Shiba et al., SPIE Vol. 470 Optical Microlithography III, pages 233-240 (1984).
Recently Wihl et al., in the U.S. patent application of which this patent application is a Continuation-In-Part application (Ser. No. 07/784,984) describes a method for the inspection of photomask substrates utilizing both reflected and transmitted light and suggested the use of both to classify defects.
It would be advantageous to extend the use of both reflected and transmitted light to obviate the need for using a die-to-die comparison for the detection of particles. It would also be advantageous to have a system that can also identify the location of the defect on the substrate without using a die-to-die comparison. Various embodiments of the present invention provide such a system and method.